Green tea leaf (as picked) contains colourless polyphenols known as catechins. The four major catechins in green tea leaf are epicatechin (EC) and epigallocatechin (EGC) and the gallated forms of these catechins (bearing a gallic acid (GA) residue), epicatechin-3-gallate (ECG) and epigallocatechin-3-gallate (EGCG). The structures of these compounds are shown in FIG. 1.
During oxidative fermentation of green leaf to produce black tea (either solid state fermentation to produce black leaf or slurry fermentation to produce black tea extracts) the catechins undergo oxidative biotransformations, through their quinones, into dimeric compounds known as theaflavins (TFs) and higher molecular weight compounds known as thearubigins (TRs). TFs and TRs are responsible for the orange and brown colours of black tea infusions and products as well as making significant contributions to the astringency and body of the made tea. TRs are larger in size and darker in colour than TFs. The oxidative polymerisations are a combination of biochemical oxidations mediated by polyphenol oxidase and/or peroxidase enzymes present in the leaf and chemical reactions of reactive species. TFs include theaflavin (TF or TF1) and a range of related gallated derivatives, the structures of some of which are shown in FIG. 2.
Theaflavin and theaflavins have been recognised as providing the "brightness" and "briskness" quality attributes of tea. They also affect tea colour. Most theaflavins have antioxidant properties and are therefore of great interest to the food and health industries.
U.S. Pat. No. 5,532,012 (Ballentine et al) describes extraction from tea cream (cold water insoluble material resulting from infusion of black tea with hot water) of a mixture of theaflavins closely approximating the natural mixture of theaflavins found in tea cream. The mixture of theaflavins extracted comprises theaflavin, theaflavin monogallates and theaflavin digallate.
The present invention is concerned with producing theaflavin by slurry fermenation and cold water soluble tea powders that have enhanced theaflavin levels.